This invention relates generally to a bridge mix such as for use in an initiator such as for an inflator device for an inflatable restraint system. More particularly, the invention relates to a low density slurry bridge mix that may more readily adhere to a bridgewire.
It is well known to protect a vehicle occupant using a cushion or bag, e.g., an “airbag cushion” that is inflated or expanded with a gas when a vehicle experiences a sudden deceleration, such as in the event of a collision. Such airbag restraint systems normally include: one or more airbag cushions, housed in an uninflated and folded condition to minimize space requirements; one or more crash sensors mounted on or to the frame or body of a vehicle to detect sudden deceleration of the vehicle; an activation system electronically triggered by the crash sensors; and an inflator device that produced or supplies a gas to inflate the airbag cushion. In the event of a sudden deceleration of the vehicle, the crash sensors trigger the activation system which in turn triggers the inflator device which begins to inflate the airbag cushion in a matter of milliseconds.
Many types of inflator devices have been disclosed in the art for inflating one or more inflatable restraint system airbag cushions. Such inflator devices typically include: an igniter assembly or initiator; multiple combustible pyrotechnic compositions; and a housing for containing the initiator and the one or more pyrotechnic compositions. The initiator generally includes a header having one or more electrically conductive pins connected to a bridgewire to form a closed electrical circuit. The bridgewire is maintained in intimate contact with an electrically ignitable initiator charge or bridge mix that is held or maintained within a welded metal sleeve or charge holder. In the event of a collision, the activation system directs an electrical current through the conductive pins of the initiator to the bridgewire. The bridgewire ignites the initiator charge or bridge mix which in turn ignites an associated pyrotechnic composition such as an igniter composition or a gas generant composition to begin production of inflation gas and, thereby, inflation of an associated or corresponding airbag cushion.
Currently available automotive initiators commonly include a glass-to-metal sealed header or bridge, a welded bridgewire on a ground surface or other resistive heating device such as a thin film resistor or semi-conductor bridge, and a metal-oxidant pyrotechnic mixture or bridge mix. Typical bridgewires used in such automotive initiators have a relatively fine diameter, e.g., on the order of about 15 microns to about 40 microns. Such initiators also typically include a metal sleeve or charge holder adjoined to the header to contain the metal-oxidant bridge mix.
One metal-oxidant bridge mix that has been used in such automotive initiators includes a pyrotechnic composition containing a mixture of zirconium metal and potassium perchlorate with a polymeric binder, generally referred to as a ZPP mixture or composition. In order for such ZPP mixtures to function properly, the mixture is typically deposited on the bridge either as a granular powder or as a slurry and then compacted to a high percentage of its theoretical density e.g., about 80 to 95 percent of theoretical density. In general, the zirconium/potassium perchlorate or ZPP mixture is compacted to such a high percentage of its theoretical density because lower density ZPP mixtures may not conduct heat generated by the bridge sufficiently fast enough to meet stringent industry initiator sensitivity requirements and therefore may not reliably ignite an associated pyrotechnic composition to initiate inflation of an associated airbag cushion.
Typically, such currently available automotive initiators are produced by welding a charge holder to a header that includes a welded bridgewire, loading a ZPP mixture into the charge holder over the bridgewire and then pressing or compacting the ZPP mixture onto the bridgewire. In order to protect and support the bridgewire during the pressing or compacting process, the header is typically ground flat to within a small fraction of the diameter of the bridgewire. Pressing the bridgewire against this flat surface also provides a conductive heat sink that assists in lowering the sensitivity of the initiator. Generally, competitive conditions within the automotive safety restraint system industry make it desirable to produce such initiators at a lower cost than current designs, especially for the seat belt pretensioner market. However, manufacture or production steps such as providing and welding a charge holder to the header, properly grinding the header to provide support and protection to the relatively fragile bridgewire, and compacting the bridge mix to a desirably density add expense to the manufacturing process.
In view of the above, there is a need and a demand for an initiator that is more cost-effective to manufacture than commercially available initiators. In particular, there is an need or demand for a low density slurry bridge mix for use in such an initiator that is effective to reproducibly and reliably ignite an associated pyrotechnic composition. There is a further need or demand for a low density slurry bridge mix that may more readily adhere to a raised bridgewire thereby eliminating the need for providing and welding a charge holder to a header that has been ground to provide support and a heat sink for the bridgewire.